Method for manufacturing spheroidal graphite cast iron

ABSTRACT

A method for manufacturing spheroidal graphite cast iron characterized by plastically deforming spherulitic graphite cast iron at a draft of 30 to 80% and then heat-treating for obtaining high tensile strength.

United States Patent Hanai et al.

[451 July 8,1975

METHOD FOR MANUFACTURING SPHEROIDAL GRAPHITE CAST IRON Inventors: Kenichi Hanai, Urawa; Tsukasa Fukumura, Okaya, both of Japan Assignees: Nippon Kinzoku Co. Ltd.; Teikoku Piston Ring Co. Ltd., both of Tokyo, Japan Filed: May 7, 1973 Appl. No.: 358,028

U.S. Cl. 148/12 R; 148/35; 75/123 CB Int. Cl C21d 5/00 Field of Search 148/2, 12, 139, 35;

References Cited UNITED STATES PATENTS Bonte 148/35 Schermer 148/35 Forbes et a1. 148/35 Bonte 148/35 Heine et al. 75/123 CB Primary Examiner-W. Stallard ABSTRACT 4 Claims, No Drawings METHOD FOR MANUFACTURING SPHEROIDAL GRAPHITE CAST IRON BACKGROUND OF THE INVENTION The present invention relates to a high tensile strength spheroidal graphite case iron possessing strength comparable to the special steel in spite of the fact that it belongs to the cast iron.

Generally as high tensile materials are used special steels in most cases. These special steels, however, do not exhibit precipitation of graphite and cannot therefore retain oil or provide self-lubrication, so that the use of them is restricted for high speed and high load operation. It is also one of the disadvantages that consideration must be given to the vibration absorption in case such steels are used as a machine element, because of their low damping capacity.

Cast iron is generally superior to the steels in wear resistance or damping capacity. However, even spherulitic graphite cast iron considered excellent in strength and toughness among cast irons has a tensile strength as low as 70 to 90 kg/mm It is quite seldom to obtain a strength exceeding 100 kg/mm even in case choice of material is melted and, after graphitization treatment, quenched and tempered. Accordingly, the increase of wall-thickness is unavoidable when cast iron is used in place of steels, bringing about the increase of weight and size of a machine part and, more seriously, its fatal drawback lies inherently in the difficulty to obtain light section casting.

SUMMARY OF THE INVENTION The present invention is intended for eliminating such disadvantages to provide a high tensile strength spheroidal graphite cast iron comparable to special steels, superior to the conventional rolled cast iron or heat-treated special cast iron, retaining such characteristics inherent in cast iron as wear resistance and high damping capacity.

The present invention is to provide a high tensile strength spheroidal graphite cast iron possessing a tensile strength higher than 100 kg/mm by plastically deforming at a draft of 30 to 80% a spheroidal graphite cast iron having a normal structure of graphite spherical or spheroidal in form (hereinafter referred to simply as spheroidal graphite cast iron) and by subsequently applying such heat treatment as quenching and tempering.

DETAILED DESCRIPTION OF THE INVENTION The reason for limiting the draft of plastic deformation of spheroidal graphite cast iron to 30 to 80% in the present invention is that the mutual dependent effect of plastic deformation and the subsequent heat treatment cannot develop fully in case of draft under 30% and that workability falls down in case of draft over 80%. With such restriction, as described in the examples later described, a cast iron possessing a tensile strength from l70to 190 kg/mm can be obtained by subjecting a material to quenching from above A transformation point and later to tempering, following the plastic deformation; and in case toughness is needed, a cast having a tensile strength from 160 to 170 kg/mm and proper toughness can be obtained by isothermal heat treatment after the plastic deformation.

Accordingly, a cast iron possessing an epochal tensile strength can be obtained unexpectedly of conventional cast iron according to the present invention. The advantages of this invention include not only reduction of member weight to make possible the design of compact machinery but usability for members requiring high levels of mechanical strength.

It has long been admitted that a light section is very difficult to cast. Even if a light section material is obtained by casting, it would take a large amount of cost for surface machining amounting to more than half of the total production cost. In case of the present invention, even if a plate material as thin as l to 3 m/m, for example, is desired, the cast iron according to the present invention may be plastically deformed close to a desired thickness and the machining allowance is thereby reduced to the atmost (for instance, 0.05 to 0.02 m/m thus markedly lessening the production cost as compared with the conventional cast iron. Furthermore, either one or both niobium and tantalum are contained in the cast material according to the present invention while molten, wear resistance increases and hardenability decreases and it is confirmed that machinability is improved consequently.

In case either one or both niobium and tantalum are contained in a quantity of 0.05 to 2.5%, very minute and hard globular carbides and nitrides (globule diameter 1 to 5p. of those elements are distributed precipitating all over the matrix independently of grain boundaries at a concentration over lOO/mm". Consequently, in case the material is used as a sliding member, the contact face pressure is uniformly distributed and develops a useful bearing effect. When the material is cooled before the above carbides do not dissolve sufficiently into austenite, the hardenability decreases and even a thin plate material cannot show supercooling effect and may be easily worked to its advantage. For instance, a niobium containing spheroidal graphite cast iron is rolled, then heated to 930C and held for 30 minutes, and subsequently cooled; however, it does not precipitate martensite in the matrix and is easily worked. In case niobium is not contained, however, the material less than 3.2 mm in wall thickness shows hardenability effect and exhibits high hardness when naturally cooled, and needs annealing when machined.

In the present invention, it is preferable to improve wear resistance that the number of precipitations of globular carbide and nitride of niobium and tantalum is larger than lOO/mm It is further preferable that either of niobium and tantalum or both are added in a quantity of 0.05 to 2.5%. The reason for establishing the lower and upper limits is that niobium or tantalum addition less than 0.05% is not sufficient for producing the globular carbides and nitrides of niobium and tantalum over mm in number; the upper limit is set to 2.5% because niobium and tantalum are expensive elements and it is desired that their content is low as long as wear resistance can develop. The globular carbides and nitrides of niobium or tantalum exhibit roughly equal nature and distribute uniformly and minutely all over the matrix. Accordingly, the effect of globular carbide and nitride precipitates on the wear resistance is not appreciably different whether either or both niobium and tantalum are added.

The cast iron according to the present invention is explained hereafter in reference with practical examples.

EXAMPLE 1 According to a known method, a spherical graphite cast iron having a main composition of 3.70% C, 2.64% Si, 0.29% Mn, 0.10% P and 0.012% S is formed into a plate of 20 m/m (T) X 100 m/m (W) X 600 m/m (L) and surface-machined to 15 m/m (T) X 60 m/m (W) x 500 m/m (L). After preliminary heating for 1 hour at 1,050C, the specimen is immediately hot-rolled and the thickness is reduced to 10.5 m/m (draft 30%), to 7.5 m/m (draft 50%), to 4.5 m/m (draft 70%), and to 3.0 m/m (draft 80%). After hot rolling, specimens of 2.0 m/m (T) X 20 m/m (W) X 180 m/m (L) at each draft are prepared by machining, and heated in a salt bath to 890C and 850C and held at such temperatures for 1 hour and then oil-quenched, and subsequently tempered for 30 minutes at 300C. The tensile strengths of the specimens are measured and the result is shown in Table 1.

Note: The above values are mean values of tensile strengths measured twice in the direction parallel to rolling.

EXAMPLE FOR REFERENCE.

The same specimens as tested in Example 1 are tested for measuring tensile strength in the direction parallel to rolling when only hot-rolled omitting quenching and tempering treatment. The result is shown in Table 2.

Table 2 Draft 30 50 60 70 80 Tensile strength 76.0 80.1 84.5 92.3 101.0 100.0 (kg/mm Note: The values in Table 2 are mean values of results measured twice.

As clearly seen in the above Example and Example for reference, high tensile strength is not obtained when merely plastically deformed or when merely quenched and tempered, but a really high tensile strength is obtained only when both treatments are carried out.

EXAMPLE 2 According to a known method, a spherical graphite cast iron having a main composition of 3.38% C, 3.47% Si, 0.41% Mn, 0.08% P, 0.011% S, 0.14% Cr, and 0.32% Nb is formed into a plate of m/m (T) X 100 m/m (W) X 600 m/m (L) and surface-machined to 6.0 m/m X 85 m/m (W) X 500 m/m (L). After preliminary heating for 1 hour at 1,000C, the specimen is immediately hot-rolled and the thickness is reduced to 4.55 m/m (draft 24%), to 3.5 m/m (draft 42%), and to 2.7

Table 3 Austenitizing temperature (C) 930 850 Draft 55 169.5 kg/mm 172.3 kg/mm As clearly seen from the above Table, the cast iron according to the present invention exhibits a high level of tensile strength comparable to special steels.

EXAMPLE 3 According to a known method, a spherical graphite cast iron having a main composition of 3.57% C, 2.62% Si, 0.54% Mn, 0.12% P, 0.011% S, 0.18% Cr, and 0.24% Nb is formed into a plate of 10 m/m (T) X 100 m/m (W) X 500 m/m (L) and surface-machined to 6.0 m/m (T) X m/m (W) X 400 m/m (L). After annealing for 2 hours at 700C, the specimen is cold-rolled while intermediately annealing and the thickness is reduced to 4.4 m/m (draft 27%), to 3.2 m/m (draft 47%), to 2.2 m/m (draft 63%), and to 1.6 m/m (draft 73%). The specimen is subsequently subjected to normalizing treatment through heating at 930C for 30 min. followed by air cooling. A specimen for abrasion test is prepared for each draft. The specimen is tested for wear resistance by Ogoshi quick abrasion test machine. The result is shown in Table 4.

The test conditions are as follows: sliding distance: 200 m, Final load: 6.4 kg Sliding block: chromium-plated rotor Fixed block: specimen at various drafts Sliding velocity: 1.02 m/sec, 4.00 m/sec, 7.15 m/sec Lubrication: dry abrasion Table 4 Specimen thickness Draft Direction parallel to rolling m/m 1.02 m/sec 4.0 m/sec 7.15 m/sec Specimen thickness Draft Direction perpendicular to rolling m/m 1.02 m/sec 4.0 m/sec 7.15 m/sec For comparison, some special steels and conventional cast irons are tested for wear resistance under the same condition, and the result is shown in Table 5.

cast iron or piston ring) Note: Tests at a low speed range (1.02 mlsec) fail owing to the occurrence of seizure.

The values shown in Tables 4 and 5 indicate worn volumes (mm) of the abrasion trace on the specimen surface tested 4 times for each specimen.

From these results, it is seen that the cast iron according to the present invention exhibits wear resistance superior to those of steels and conventional cast irons and the wear resistance does not show anisotropy whether it is parallel or perpendicular to the rolling direction.

High tensile strength spheroidal graphite cast iron according to the present invention can exhibit high tensile strength far greater, by applying both plastic deformation and heat treatment to those of ordinary composition or containing either or both niobium and tantalum,

than by applying only plastic deformation or heat treatment, and also may possess characteristics comparable to special steels as well as excellent properties inherent in cast iron such as oil retentivity, wear resistance, and high damping capacity.

Accordingly, this cast iron finds new fields of application unforeseeable for the conventional cast irons and can contribute much to industry, making the equipment and machinery lighter in weight and reducing cost by use of this cast iron.

What we claim is:

l. A method for manufacturing spheroidal graphite cast iron comprising applying hot plastic deformation treatment to a spherical graphite cast iron at a reduction ratio of 30 to and subsequently subjecting the said material to heat treatment to obtain high tensile strength.

2. A method for manufacturing spheroidal graphite cast iron as recited in claim 1, wherein the said heat treatment comprises quenching and tempering.

3. A method for manufacturing spheroidal graphite cast iron as recited in claim 1, wherein the said heat treatment comprises austempering.

4. A method for manufacturing spheroidal graphite cast iron as recited in claim 1, wherein the said spheriodal graphite cast iron contains 0.05 to 2.5% of at least one element selected from niobium and tantalum. 

1. A METHOD FOR MANUFACTURING SPHEROIDAL GRAPHITE CAST IRON COMPRISING APPLYING HOT PLASTIC DEFORMATION TREATMENT TO A SPHERICAL GRAPHITE CAST IRON AT A REDUCTION RATIO OF 30 TO 80% AND SUBSEQUENTLY SUBJECTING THE SAID MATERIAL TO HEAT TREATMENT TO OBTAIN HIGH TENSILE STRENGTH.
 2. A mEthod for manufacturing spheroidal graphite cast iron as recited in claim 1, wherein the said heat treatment comprises quenching and tempering.
 3. A method for manufacturing spheroidal graphite cast iron as recited in claim 1, wherein the said heat treatment comprises austempering.
 4. A method for manufacturing spheroidal graphite cast iron as recited in claim 1, wherein the said spheriodal graphite cast iron contains 0.05 to 2.5% of at least one element selected from niobium and tantalum. 